A multiple channel switching regulator system includes multiple switching regulator channels, each of which is capable of converting from an input voltage above, below, or equal to the controlled output voltage, respectively performing buck mode regulation, boost mode regulation, or buck-boost mode regulation.
For example, a multiple channel switching regulator system may have a cascaded arrangement, in which the first switching regulator channel converts an input voltage into an intermediate voltage. The next switching regulator channel converts the intermediate voltage to the output voltage or to another intermediate voltage. Hence, the cascaded multiple channel arrangement performs multiple conversion steps to eliminate the need for a single converter that converts, for example, a very high input voltage into a very low output voltage. Each regulator in the cascaded multiple channel arrangement maintains a relatively low step-down or step-up ratio running at high efficiency while maintaining a reasonable duty cycle. By contrast, a regulator performing a single step conversion must run at a very narrow duty cycle compromising component size, efficiency and transient response.
Another example of a multiple channel switching regulator system is a multiple input arrangement capable of operating with multiple input power supply sources. Each switching regulator channel is supplied with a respective input voltage to produce the regulated output voltage that may be common for the system or individual for each channel.
Switching regulator channels in a typical multiple channel system operate at a constant switching frequency common for all channels. The constant switching frequency arrangement minimizes output ripple amplitude, and allows inductor and capacitor values to be chosen with a precise operating frequency in mind. Also, the constant frequency operation keeps noise generated by the system in a known frequency band.
FIG. 1 illustrates a typical step-down switching regulator channel 10 including a step-down switching regulator 12 that may include an internal switch controlled to provide voltage regulation, and switch control circuitry. External components connected to the step-down switching regulator 12 include an inductor L1 coupled to the input voltage VIN when the switch is on and disconnected from Vin when the switch is off. Also, the external components include a diode D1, such as a schottky diode, coupled to the inductor terminal to provide a path for the inductor current when the switch is off. A capacitor C1 and a diode D2 may be arranged to provide boosted drive for the switching regulator channel. A capacitor C2 may be coupled to the inductor terminal and charged when the switch is on. The current through the inductor ramps up when the switch is on, and ramps down when the switch is off. The size of the external components, along with the output voltage ripple, decreases with the increase in the switching frequency of the regulator. The maximum constant switching frequency of a non-synchronous step-down switching regulator can be approximated as follows:
                                          fSW            ⁢                                                  ⁢            1            ⁢                          (              Hz              )                                =                                                    VOUT                +                VD                                            VIN                -                VSW                +                VD                                      ·                          1                              Ton                ⁡                                  (                  min                  )                                                                    ,                            (        1        )            where VIN and VOUT are input and output voltages of the switching regulator channel,    VD is the forward voltage drop of the diode D1,    VSW is the voltage drop of the internal switch, and    Ton(min) is the minimum on time period of the regulator.
FIG. 2 illustrates a typical step-up switching regulator channel 20 including a step-up switching regulator 22 that may include an internal switch controlled to provide voltage regulation, and switch control circuitry. The step-up switching regulator 22 is connected to external components including an inductor L11, a diode D11 and a capacitor C11. When the switch is on, the input voltage VIN is forced across the inductor L11 causing the current through the inductor to ramp up. When the switch is off, the decreasing inductor current provides forward biasing of the diode D11 allowing the capacitor C11 to charge up to the output voltage VOUT.
The maximum constant switching frequency of a non-synchronous step-up switching regulator can be approximated as follows:
                                          fSW            ⁢                                                  ⁢            2            ⁢                          (              Hz              )                                =                                    (                              1                -                                                      VOUT                    -                    VIN                    +                    VD                                                        VIN                    -                    VSW                    +                    VD                                                              )                        ·                          1                              Toff                ⁡                                  (                  min                  )                                                                    ,                            (        2        )            where VIN and VOUT are input and output voltages of the switching regulator,    VD is the forward voltage drop of the diode D1,    VSW is the voltage drop of the internal switch, and    Toff(min) is the minimum off time period of the regulator.
To minimize size of the external components and minimize the output voltage ripple, constant switching frequency of all channels in a typical multiple channel switching regulator system is determined by the maximum switching frequency of a channel having the lowest maximum switching frequency determined by the equations (1) and (2). This constraint determines the size of the external component for all channels.
FIG. 3 illustrates operation of an exemplary conventional 2-channel switching regulator operating at 1.5 MHz constant frequency, where channel 1 steps down a 24V input voltage to a 5V output voltage (VOUT1), and channel 2 steps down the 5V output voltage to a 1.8V output voltage (VOUT2). The diagram in FIG. 3 shows switch voltages SW1 and SW2 and output voltages Vout1 and Vout2 for channels 1 and 2, respectively. Due to the minimum on time restriction and high input to output voltage ratio on channel 1, 1.5 MhZ constant frequency operation cannot be maintained. To maintain output regulation, channel 1 switches to pulse skipping mode resulting in high output voltage ripple. To avoid pulse skipping mode and achieve the lowest possible output ripple, the overall system switching frequency must be reduced. To preserve the performance of the original system, external component values must be increased in the reduced frequency system which results in an increase in the overall system footprint. The alternative to reducing the system frequency is to limit the input to output voltage ratio of the switching regulator channels.
Therefore, there is a need for circuitry and methodology that would generate the smallest overall footprint for a multiple channel regulator system by achieving the highest possible constant frequency operation for each individual channel regardless of either channel or system input to output ratios, or other factors such as temperature which effect minimum on time restrictions of the individual channel.